J. Org. Chem. 1997, 62, 4085-4087
4085
Solid-Supported Heterocumulenes: Preparation and Crystal Structure of Azaaplysinopsins Jean M. Chezal,1a Gre´gory Delmas,1a Sylvie Mavel,1b Hamed Elakmaoui,1c Jacques Me´tin,1a Anna Diez,1d Yves Blache,1c Alain Gueiffier,1b Mario Rubiralta,1d Jean C. Teulade,1a and Olivier Chavignon*,1a De´ partement d’Analyse Structurale et de Pharmacologie, Faculte´ de Pharmacie, 28, Pl. H. Dunant, BP 38, 63001 Clermont-Fd Cedex 1, France, Laboratoire de Chimie The´ rapeutique, Faculte´ de Pharmacie, 34 Avenue Monge, 37200 Tours, France, Laboratoire de Chimie Organique, Faculte´ de Pharmacie 15, Avenue Ch. Flahault, 34060 Montpellier, France, and Laboratori de Quı´mica Orga` nica, Facultat de Farma` cia, Universitat de Barcelona, 08028-Barcelona, Spain Received November 20, 1996X
Annulation of carbodiimides 6a,b, on alumina solid support, selectively gives (Z)-hydantoins 8a,b determined on the basis of X-ray analysis while thermal cyclization affords, after purification, 8a,b with imidazolones 10a,b. In the imidazopyrimidine series, thermal reaction of heterocumulenes 6d yields, via a Dimroth rearrangement, the dipyridoimidazolic compound 11. In all cases, mechanisms for the observed cyclizations have been proposed. Introduction Aplysinopsin (1) (Figure 1), isolated from the sponge Aplysinopsis reticulata (Dictyoceartida),2 has been shown to be active as a specific cytotoxin of cancer cells3 and to affect neurotransmission.4 The classical approach to the synthesis of C-5 unsaturated hydantoins such as 2 is based on the coupling of an aromatic aldehyde and an appropriately substituted hydantoin. However, poor yields, purification difficulties, and the formation of mixtures of E and Z isomers are generally encountered.5 Such inconveniences have been circumvented by the development of a tandem Staudinger/aza-Wittig reaction followed by electrocyclic ring closure.6 Such methodology allows the formation of nitrogenated heterocycles from heterocumulenes which in turn are easily available from iminophosphoranes. Using this route, an elegant synthesis of several aplysinopsin-type alkaloids has been accomplished by Molina and co-workers.7 In the course of our studies on the reactivity of nitrogen bridgehead azaindolizines, and in view of the pharmacological8 and theoretical9 interest of azaindole structures, we embarked on the development of a model * Author to whom correspondence should be addressed. Fax: (33) 04 73 27 77 27. E-mail:
[email protected] X Abstract published in Advance ACS Abstracts, March 15, 1997. (1) (a) Clermont-Ferrand. (b) Tours. (c) Montpellier. (d) Barcelona. (2) Kazlauskas, R.; Murphy, P. T.; Quinn, R. J.; Wells, R. J. Tetrahedron. Lett. 1977, 61. (3) Hollenbeak, K.; Schmitz, F. J. Lloydia 1977, 40, 479. (4) Backer, J. T.; Wells, R. J. In Natural Products as Medicinal Agents, Beal, J. L., Reinhard, E., Eds.; Hippokrates Verlag: Stuttgart, 1981; pp 299-303. (5) (a) Ware, E. Chem. Rev. 1950, 46, 403. (b) Bateman, J. H. KirkOthmer Encyclopedia of Chemical Technology; Wiley-Interscience: New York, 1978; Vol. 12, p 692. (c) Djura, P.; Faulkner, D. J. J. Org. Chem. 1980, 45, 735. (d) Guella, G.; Mancini, I.; Zibrowins, H.; Pietra, F. Helv. Chim. Acta 1988, 71, 773. (e) Guella, G.; Mancini, I.; Zibrowius, H.; Pietra, F. Helv. Chim. Acta 1989, 72, 1444. (6) Molina, P.; Vilaplana, M. J. Synthesis 1994, 1197. (7) Molina, P.; Almendros, P.; Fresneda, P. M. Tetrahedron 1994, 50, 2241. (8) Arbilla, S.; Allen, J.; Wick, A.; Langes, S. Eur. J. Pharmacol. 1986, 130, 257. (9) (a) Hand, E. S.; Paudler, W. W. J. Org. Chem. 1975, 40, 2916. (b) Teulade, J. C.; Gueiffier, A.; Viols, H.; Chapat, J. P.; Grassy, G.; Perly, B.; Dauphin, G. J. Chem. Soc., Perkin Trans. 1 1989, 1895. (c) Chavignon, O.; Teulade, J. C.; Madesclaire, M.; Gueiffier, A.; Blache, Y.; Viols, H.; Chapat, J. P. J. Heterocycl. Chem. 1992, 29, 691. (d) Diez, A.; Mavel, S.; Teulade, J. C.; Chavignon, O.; Sinibaldi, M. E.; Troin, Y.; Rubiralta, M. Heterocycles 1993, 36, 2451.
S0022-3263(96)02174-3 CCC: $14.00
Figure 1.
system of imidazo[1,2-a](di)azines which may increase greatly the pharmacological profile.10 We reported recently a highly effective method for the synthesis of azacarboline and -aplysinopsin mimic structures from heterocumulenes.11 We describe now the synthesis of hydantoins 8, potential selective competitive NMDA antagonists,12 via stereospecific ring-opening/ring-closure of an azalactone obtained from an alumina-supported heterocumulene. Results and Discussion The 3-formyl derivative 3a9 was condensed with ethyl azidoacetate in the presence of sodium ethoxide at -30 °C to give azidovinyl 4a (νazido 2050 cm-1) in 78% yield. Measurement of the long-range 13C-1H coupling constant between the olefinic proton and the carbonyl carbon in the coupled 13C NMR spectra proved to be diagnostic of a (Z) arrangement.13 The preparation of iminophosphorane 5a was accomplished by Staudinger’s reaction of 4a with triphenylphosphine in dichloromethane at room temperature in 81% yield (Scheme 1). Spectral and analytical data were consistent with the identity of compound 5a and the (Z) configuration assignment (3JH-Cβ,CO ) 3.5 Hz). An aza-Wittig type reaction of iminophosphorane 5a with aliphatic or aromatic isocyanates in dry toluene at (10) (a) Chermann, J. C.; Gruest, J.; Montagnier, L.; Wendling, F.; Tambourin, P.; Perrin, M.; Pochon, F.; Ducrocq, C.; Rivalle, C.; Bisagni, E. C. R. Seances Acad. Sci., Ser. D 1977, 285, 945. (b) Marsais, F.; Pineau, P.; Nivolliers, F.; Mallet, M.; Godard, A.; Quequiner, G. J. Org. Chem. 1992, 57, 565. (11) Chavignon, O.; Teulade, J. C.; Roche, D.; Madesclaire, M.; Blache, Y.; Gueiffier, A.; Chabard, J. L.; Dauphin, G. J. Org. Chem. 1994, 59, 6413. (12) (a) Hamilton, S. G.; Huang, Z.; Yang, X. J.; Patch, R. J.; Narayanan, B. A.; Ferkany, J. W. J. Org. Chem. 1993, 58, 7263. (b) Cordi, A.; Sun, E. US Patent 5, 252, 563. (13) Vo¨geli, U.; Von Philipsborn, W.; Nagarajan, K.; Nair, M. D. Helv. Chim. Acta 1978, 61, 607.
© 1997 American Chemical Society
4086 J. Org. Chem., Vol. 62, No. 12, 1997
Chezal et al.
Scheme 1
Scheme 2
room temperature for 5-19 h gave the corresponding carbodiimides 6a,b, isolated as yellow plates. Since, according to their usual behavior, indole carbodiimides are expected to give the corresponding fused pyridine or hydantoin annulation, our attention was turned to the reactivity of 6a,b. Surprisingly, when carbodiimides 6a,b were heated at 110 °C, and even when conditions were forced (160 °C, o-Cl2C6H4), no evidence of the annulation was observed. However, when a toluene solution of 6a or 6b was heated at 110 °C for 3 h and the crude reaction product was chromatographed on alumina (CH2Cl2MeOH, 98:2), azaaplysinopsins 8 were obtained together with compounds 10 (8a:10a ) 98/2; 8b:10b ) 83/17). Heterocumulenes 6a,b were then refluxed in methanol and yielded the corresponding imidazolones 10a,b. The structural assignment of compounds 10a,b was proved by the presence of a methoxy group signal in their 13C NMR spectra. In contrast, when an alumina solid support was impregnated with methylene chloride solutions of heterocumulenes 6a,b, followed by elution with methanol, a rapid, stereospecific, and quantitative formation of (Z)-hydantoins 8a,b was obtained (Scheme 2). No reaction occurred when we performed the same experiment on silica gel, probably due to its acidic character. These experiments demonstrated that two different mechanisms lead to compounds 8 and 10. In the first case, heterocumulenes 6a,b undergo a ring closure across the ester functionality, followed by a spontaneous Dimroth lactone-lactam rearrangement to give hydantoins 8a,b. In the second case, the former heterocumulenes would first be transformed into 2-methylisoureas 9a,b which would lactamize to give imidazolones 10a,b. Interestingly, we noted a photoisomerization of 8a in solution. Irradiation for 4 h at rt with 365 nm light of a solution of neat (Z)-8a in (CD3)2SO was found to afford a mixture appreciably richer in the (E)-8a isomer (E/Z > 75/25). The structural constitutions of 8a,b were inferred
Figure 2. ORTEP drawing of 8a; the sizes of H atoms have been reduced for clarity. Selected bond lengths (Å) and torsion angles (deg) are as follows: N4-C8a, 1.390(3); C9-C1, 1.335(3); C1′-N2′, 1.387(3); C1′-C5′, 1.478(4); N4′-C5′, 1.363(2); C3-N4-C8a-N1, 0.1(3); N4-C3-C9-C1′, -176.4(3); C3′-N4′-C5′-C1′, 1.1(3).
from the 1H and 13C NMR spectral data and confirmed by the MS analysis (Experimental Section). The structure and the (Z) configuration of 8a were confirmed by its crystallographic data.14 A projection of the molecule obtained with the ORTEP program15 is given in Figure 2. The angle between the plane of the imidazopyridine ring and the hydantoin group was 2.2°, which proves the near planarity of the molecule. Bond distances and angles and torsion angles were mostly those expected from the hybridization of the atoms. The other angles and bond distances were similar to those of other methylene hydantoins recently reported.16 The bond lengths measured in the imidazopyridine system were consistent with those of other compounds in the same series.17 Subsequent to the demonstration of the viability of the intramolecular annulation described above, and in order to further explore the synthetic potential of the aluminasupported reaction, we pursued its application to the imidazopyrimidine ring system. Thus, alumina-supported reaction of iminophosphorane 5b with isocyanates yielded imidazolidindiones 8c,d in 49 and 45% yields, respectively. The presence of an imidazolidindione functionality at the imidazopyrimidine C-3 position was based on the 1H and 13C NMR chemical shifts. This methodology avoids the Dimroth rearrangement and the consequent pyridinization which generally occurs in these heterocyclic structures.11 The latter reaction is exemplified by the formation of 11 due to thermal reaction of 5b with C6H5NCO (Scheme 3). In conclusion,we have described an alternative methodology for obtaining the annulation of hydantoins, based on an alumina-supported reaction of heterocumulenes. A mechanistic explanation is given. The methodology proves to be general for the preparation of azaaplysinopsins and has been applied here to the synthesis of structures 8a-d. (14) The authors have deposited atomic coordinates for this structure with the Cambridge Crystallographic Data Centre. The coordinates can be obtained, on request, from the Director, Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge, CB2 1EZ, UK. (15) (a) Johnson, C. K. ORTEP-II: A Fortran Thermal Ellipsoid Plot Program for Crystal Structure Illustrations, ORNL-5138, 1976. (b) Main, P.; Fisker S. J.; Hull, S. E.; Lessinger, L.; Germain, G.; Declercq, J. P.; Woolfson, M. M.; MULTAN 11/82. A System of Computer Programs for the Automatic Solution of Crystal Structures from X-ray Diffraction Data, University of York England and University of Louvain, Belgium, 1980. (c) MolEN, An Interactive Structure Solution Procedure, Enraf-NONIUS, Delft, The Netherlands, 1990. (d) International Tables for X-ray Crystallography, Kynoch Press: Birmingham, England; Vol. 4, 1974. (e) Crystal data for 8a with Mo KR radiation: a ) 14.195(3), b ) 12.306(1), c ) 7.096(2), R (deg) ) 90, β (deg) ) 90, γ (deg) )111.24(1), monoclinic, P21/b, 1505 observed reflections with I > 3 σ(I). The final R factors were R ) 0.049 and RW ) 0.046. (16) Gallucci, J.; Mathur, N.; Shechter, H. Acta Crystallogr., Sect. C 1992, 48, 477. (17) Teulade J. C.; Escale, R.; Rossi, J. C.; Chapat, J. P.; Grassy, G.; Payard, M. Aust. J. Chem. 1982, 35, 1761.
Solid-Supported Heterocumulenes
J. Org. Chem., Vol. 62, No. 12, 1997 4087
Scheme 3
alumina column. Elution with CH2Cl2 then with CH2Cl2/ methanol (80/20) afforded compound (Z)-5-[(imidazo[1,2-a]pyridin-3-yl)methylidene]-3-ethyl-1H-imidazolidine-2,4-dione (8a) as yellow prisms (65%): mp > 260 °C; IR (KBr) 3120, 1730, 1670, 1640 cm-1; 1H NMR (DMSO-d6) δ 1.15 (t, 3H, J ) 7 Hz), 3.52 (q, 2H, J ) 7 Hz), 6.99 (s, 1H), 7.08 (t, 1H, J ) 7 Hz), 7.42 (ps t, 1H), 7.68 (d, 1H, J ) 9 Hz), 8.37 (s, 1H), 8.87 (d, 1H, J ) 7 Hz), 10.55 (s, 1H); 13C NMR (DMSO-d6) δ 13.4, 32.9, 94.9, 113.3, 117.2, 119.2, 124.0, 125.2, 126.2, 135.9, 145.9, 154.5, 163.3; m/z (%): 256 (M+, 100), 157 (87); UV (MeOH): 353, 272, 224, 203 nm. Anal. Calcd for C13H12N4O2: C, 60.94; H, 4.69; N, 21.87. Found: C, 61.05; H, 4.65; N, 21.92. (E)-5-[(Imidazo[1,2-a]pyridin-3-yl)methylidene]-3-ethyl1H-imidazolidine-2,4-dione (8a). Irradiations under UV (λ ) 365 nm) of (Z)-8a (40 mg, 0.15 mmol) in (CD3)2SO were carried out in a 5 mm NMR tube for 4 h, monitoring the transformations by 1H and 13C NMR. Clean (Z)-8a f (E)-8a isomerization was observed, without byproducts. The ratio of the mixture was (E)/(Z) > 75/25; (E)isomer 1H NMR (DMSOd6) δ 1.15 (t, 3H, J ) 7 Hz), 3.52 (q, 2H, J ) 7 Hz), 6.71 (s, 1H), 7.05 (ps t, J ) 6 Hz, 1H), 7.37 (ps t, 1H), 7.65 (d, 1H, J ) 9 Hz), 8.50 (d, 1H, J ) 7 Hz), 8.75 (s, 1H), 10.50 (s, 1H); 13C NMR (DMSO-d6) δ 13.3, 32.7, 99.5, 113.29, 117.3, 119.2, 124.0, 125.2, 126.2, 137.4, 145.8, 152.8, 161.7; m/z (%): 256 (M+, 100), 157 (87). 5-(Imidazo[1,2-a]pyridin-3-yl]methylidene)-3-ethyl-2methoxyimidazolin-4-one (10a). A solution of 6a (0.46 mmol) in methanol (40 mL) was heated at reflux for 4 h. After cooling, the solvent was removed off under reduced pressure, and the residue was chromatographed on an alumina column with CH2Cl2 as eluent to give the imidazoline 10a as yellow prisms (27%); mp 180-182 °C; IR (KBr) 1700, 1640, 1260 cm-1; 1H NMR (CDCl ) δ 1.25 (t, 3H, J ) 7 Hz), 3.61 (q, 2H, J ) 7 3 Hz), 4.20 (s, 3H), 6.95 (t, 1H, J ) 7 Hz), 7.11 (s, 1H), 7.28 (ps t, 1H), 7.69 (d, 1H, J ) 9 Hz), 8.42 (d, 1H), 8.61 (s, 1H); 13C NMR (CDCl3) δ 7.8, 30.2, 54.4, 108.5, 117.2, 122.3, 126.0, 128.7, 130.9, 141.3, 146.2, 153.7, 17O.8, 177.3; m/z (%): 270 (M+, 57), 156 (100), 58 (83). Anal. Calcd for C14H14N4O2: C, 62.22; H, 5.19; N, 20.74. Found: C, 61.98; H, 5.20; N, 20.79. A solution of carbodiimide 6a or 6b in toluene was heated at 110 °C for 3 h. After cooling, the residue was chromatographed on an alumina column, eluting with CH2Cl2-MeOH (98:2) to give the imidazolines 10a,b and then the hydantoins 8a,b. The ratios were 8a/10a: 98/2 and 8b/10b: 83/17. Thermal Reaction of Iminophosphorane 5b with Phenyl Isocyanate. To a solution of iminophosphorane 5b (2 mmol) in 25 mL of 1,2,4-trichlorobenzene was added phenyl isocyanate (2.2 mmol). The reaction mixture was stirred at room temperature for 3 h and then at 160 °C for 8 h. After cooling, the solution was washed with cold ethanol to remove phosphine oxide, and the resulting mixture was chromatographed on an alumina column eluting with CH2Cl2 to yield product 11 as yellow prisms (0.49 g, 72%): mp > 260 °C; IR (KBr) 3400, 1730, 1250 cm-1; 1H NMR (CDCl3) δ 1.54 (t, 3H, J ) 7 Hz), 4.51 (q, 2H, J ) 7 Hz), 7.11 (m, 2H), 7.42 (t, 2H), 8.07 (s, 1H), 8.12 (d, 2H), 8.21 (s, 1H), 8.78 (dd, 1H, J ) 7 Hz, J ) 2 Hz), 8.91 (dd, 1H, J ) 4 Hz); 13C NMR (CDCl3) δ 14.4, 61.7, 100.9, 108.8, 118.5, 122.4, 129.1, 130.1, 131.4, 133.8, 138.6, 140.0, 147.7, 150.6, 156.4, 165.9; m/z (%): 333 (M+, 94), 260 (78), 79 (100). Anal. Calcd for C18H15N5O2: C, 64.86; H, 4.50; N, 21.02. Found: C, 65.05; H, 4.49; N, 21.04.
Experimental Section18 Preparation of Ethyl r-Azido-β-(imidazo[1,2-a]pyridin-3-yl)propenoate (4a). Ethyl azidoacetate (10.38 g, 80 mmol) was added dropwise at -30 °C to a stirred solution containing sodium (0.8 g, 35 mmol) in dry ethanol (25 mL). To this solution was added dropwise a mixture of aldehyde (8 mmol) in dry ethanol (10 mL). The reaction mixture was returned to rt and stirred for 3 h. The solution was poured into aqueous saturated NH4Cl solution (100 mL) and then extracted with ether. The organic layers were dried over anhydrous Na2SO4 and evaporated in vacuo. The crude product was chromatographed on an alumina column eluting with CH2Cl2 to give 4a (78%): mp 98-100 °C; IR (KBr) 2050, 1680, 1220 cm-1; 1H NMR (CDCl3) δ 1.35 (t, 3H, J ) 7 Hz), 4.31 (q, 2H, J ) 7 Hz), 6.87 (t, 1H, J ) 7 Hz), 6.96 (s, 1H), 7.21 (ps t, 1H), 7.57 (d, 1H, J ) 9 Hz), 8.11 (d, 1H), 8.38 (s, 1H); 13C NMR (CDCl3) δ 14.1, 62.1, 108.5, 113.4, 118.0, 119.7, 122.3, 123.1, 125.9, 138.7, 146.4, 162.8. Anal. Calcd for C12H11N5O2: C, 56.03; H, 4.28; N, 27.24. Found: C, 56.19; H, 4.29; N, 27.21. Ethyl r-[(Triphenylphosphoranylidene)amino]-β-(imidazo[1,2-a]pyridin-3-yl)propenoate (5a). To a solution of vinylazide (6 mmol) in dry CH2Cl2 (20 mL) was added dropwise, at 0 °C, a solution of triphenylphosphine (1.57 g, 6 mmol) in the same solvent (20 mL). The reaction mixture was stirred at rt for 12 h, and the solvent was removed in vacuo. The crude product was purified by chromatography on alumina column, eluting with CH2Cl2 to give the iminophosphorane 5a (81%): mp 164-166 °C; IR (KBr) 1690, 1580, 1425, 1225 cm-1; 1H NMR (CDCl ) δ 0.98 (t, 3H, J ) 7 Hz), 3.88 (q, 2H, J ) 7 3 Hz), 6.83 (t, 1H, J ) 7 Hz), 6.96 (d, 1H, 4JH-P ) 7 Hz), 7.15 (ps t, 1H), 7.47 (m, 9H), 7.61 (d, 1H, J ) 9 Hz), 7.74 (m, 6H), 8.20 (d, 1H, J ) 7 Hz), 8.59 (s, 1H); 13C NMR (CDCl3) δ 14.1, 60.8, 101.9 (d, 3JP-C ) 21 Hz), 111.9, 117.8, 123.5, 123.7, 124.1, 128.3 (d, 3JP-C ) 12 Hz, 3C), 131.1, 132.3 (d, 2JP-C ) 10 Hz, 6C), 132.6 (d, 1JP-C ) 103 Hz, 3C), 135.34(d, 2JP-C ) 6 Hz), 135.6, 144.9, 166.8 (d, 3JP-C ) 7 Hz); m/z (%): 491 (M+, 42), 156 (100). Anal. Calcd for C30H26N3O2P: C, 73.32; H, 5.30; N, 8.55. Found: C, 73.35; H, 5.28; N, 8.58. Preparation of Carbodiimide 6b. To a solution of iminophosphorane (0.81 mmol) in dry toluene (50 mL) was added dropwise at 0 °C phenyl isocyanate (1.68 mmol). The solution was stirred at 0 °C for 30 min and then at rt for 5 h. The solvent was removed off under reduced pressure, and the crude product was washed with hexane to remove phosphine oxide and give 6b (56%): mp: 164-166 °C; IR (KBr) 2130, 1690, 1240 cm-1; 1H NMR (CDCl3) δ 1.39 (t, 3H, J ) 7 Hz), 4.41 (q, 2H, J ) 7 Hz), 6.96 (t, 1H, J ) 7 Hz), 7.16 (m, 1H), 7.30 (m, 6H), 7.67 (d, 1H, J ) 9 Hz), 8.22 (d, 1H), 8.65 (s, 1H); 13C NMR (CDCl ) δ 14.3, 62.3, 112.1, 113.5, 118.3, 120.5, 122.6, 3 123.3, 124.4 (2C), 125.4, 126.0, 129.4 (2C), 135.1, 138.0, 139.1, 146.7, 164.0; m/z (%): 332 (M+, 34), 156 (100), 78 (50). Anal. Calcd for C19H16N4O2: C, 68.67; H, 4.82; N, 16.87. Found: C, 68.83; H, 4.81; N, 16.88. General Procedure for the Preparation of Imidazolidine-2,4-dione 8a. The carbodiimide was poured onto an (18) For general experimental details, consult ref 11.
Acknowledgment. We express our grateful acknowledgment to Claire Lartigue for mass spectral data and Mr. Damien Canitrot and Henri Viols for skillful experimental work. We thank the University of Auvergne for financial support. Supporting Information Available: Experimental data for 4b, 5b, 6a,d, 8b-d, and 10b, and crystal data for 8a (3 pages). This material is contained in libraries on microfiche, immediately follows this article in the microfilm version of the journal, and can be ordered from the ACS; see any current masthead page for ordering information. JO962174Y
